1. Field of the Invention
This invention relates to removal of solid materials from liquid, and more particularly, to a screen for automatically removing undesired solid materials, such as non-degradable debris, from liquid flowing into a water or waste water or liquid industrial waste treatment facility.
2. Discussion of Prior Screens
Basins are used to remove certain undesired solid materials from liquid, to clarify the liquid. Basins are used, for example, in water and waste water treatment plants, and in industrial waste treatment plants. In water treatment, the water drawn from a water supply has various undesirable solid materials therein. One type of undesired material is non-settleable colloidal solid material. When mixed with chemicals, the non-settleable colloidal solid materials and the chemicals agglomerate to form solid materials which will settle from the water. In water and waste water treatment, the undesired solid materials include organic solids. In industrial processes, the undesired solid materials may include a variety of materials, such as fibers, which are not amenable to agglomeration and settling, for example.
Water, waste water, and liquid industrial wastes are treated in basins to remove such undesired solid materials, thereby making the water clear and suitable for use, reuse, or for further treatment, such as tertiary treatment. The word "liquid" as used below to describe the present invention refers to water, waste water and liquid industrial wastes.
These undesired solid materials include particles that are suspended in the liquid. Devices for removing the suspended materials are well-known. Such devices substantially reduce the flow rate of the liquid, and the particles therein, as in a very low flow, or quiescent, zone (or flow channel) in the basin. The suspended, low flow rate particles are subjected to the force of gravity and settle to the bottom of the basin as sludge.
However, the undesired solid materials to be removed from such liquid include items that are generally much larger than such suspended, settleable materials, and that are not removable by settling. Many of these larger materials are not regulated by government waste management authorities, and include, for example, (i) man-made materials such as plastics (e.g., plastic shapes for holding six cans of soda in a six-pack); (ii) large materials such as other plastic waste, containers, construction debris, and refuse carried by, but generally not suspended in, the liquid; and (iii) many other materials (such as paper towels) which are carried by the liquid, but which generally do not degrade in the manner in which biodegradable materials (such as food) degrade.
For purposes of differentiating the undesired, suspended, settleable solid materials from such other undesired, non-settleable larger materials, these undesired, suspended, settleable solid materials are referred to herein as "solids" or "settleable solids", whereas these undesired, non-settleable larger materials are referred to herein as "screenable solid materials", or "debris", because they are desirably removed by "screening" before the liquid and the settleable solids flow into the basin.
If such debris are not removed before the liquid and the solids flow into the basin, the (a) debris may interfere with the settling process by which the settleable solids are encouraged to settle to the bottom of the basin and form sludge, (b) debris in the sludge may interfere with removal of the sludge from the basin, and (c) the sludge collected from the bottom of the basin will include some of the debris. Such interference with the settling process is undesirable because less settleable solids are removed per gallon of liquid processed through the settler. Also, the sludge is temporarily stored ("inventoried") before use, such that any excess (i.e., non-settleable solid) material in the inventoried sludge increases the land area or other facilities needed to store the sludge. Further, because such debris (e.g., plastics) is not suitable for use in the manner in which the sludge is used (e.g., fertilizer for agricultural purposes), the commercial value of sludge which contains debris is greatly reduced, increasing the net cost of the liquid clarifying operation.
Accordingly, efforts have been made to provide ways of removing the debris from the liquid and the solids before the liquid and the solids flow into the basin. Applicants' studies indicate that the type of prior method or apparatus used to remove the screenable solids from the liquid and the solids seems to depend on the size of a channel (such as a pipe or rectangular concrete conduit) through which the debris, the liquid, and the solids flow on the way to the basin. While many types of facilities have been used for large size channels (e.g. those in the range of two to three feet wide and two to five feet deep), one type of facility, a chopper, has been predominately used for small size channels (e.g. those in the range of one to two feet wide and one to two feet deep). Moreover, such choppers are generally not effective, because in the process of chopping up the debris, non-settleable small pieces of the debris (e.g., small fibers) tend to stay in the flow stream of the liquid and solids, leading to the above-noted disadvantages.
Thus, although others have used fixed bar screens to prevent such debris from entering the basins through the large size channels, it appears that such fixed bar screens are much less often used for the small size channels. In such fixed bar screens, a rack holds closely spaced bars across the flow path of the channel in which the liquid, the suspended solids and the debris flow toward the basin. As the debris flow with the liquid and the suspended solids, the debris are to be caught on the front face of the bars, whereas the liquid and the suspended solids flow through openings between the bars.
Regardless of the size of the channel in which the fixed bar screens have been used, problems have been faced in the use of facilities in an attempt to remove the debris from the face of the bars in a reliable manner without human intervention on a 24 hour-a-day basis. The removal facilities tend to be structures of great height (e.g., twelve feet) above the channel, or of significant complexity, or which tend to block the flow of the liquid and the suspended solids through the openings between the bars, or which are very costly.
For example, some such removal facilities provide a rake-like structure (a "rake") having tines which can extend into the spaces between the bars. The rakes are guided relative to the bars by guides which generally are fixed, such as slots machined into a metal plate in which a guide roller rides. The slots are shaped to guide the roller, and hence the rake attached to the roller, into the channel so that the tines of the rake move toward and then along the face of the bars in a preset, or invariable, raking action. This invariable action pulls the tines across the bars to move the screenable solids up off the face of the bars and out of the channel. Among the problems faced using such rakes with fixed guides, the debris often becomes lodged in the bars so that the tines of the rake become jammed. When jammed, springs which bias the tines toward the bars are to be overridden and the rake is to move over the object that caused the jamming. In many, if not all, of such systems, once the rake starts moving under the action of the rollers riding in the machined slots in the preset raking action, except for the overcoming of the bias of the spring, the rake must complete the entire preset traverse of the bars. The reliance on the spring to release the rake in case jamming occurs does not allow the fixed bar screens to remove the cause of the jamming, or otherwise vary the sequence of rake motion.
There are generally two types of removal facilities for fixed bar screens. As noted above, some removal facilities for a fixed bar screen have tines which extend into the spaces between the bars. The bars generally extend upwardly in the channel, so that the openings between the bars also extend upwardly and facilitate upward motion of the rake for removal of the debris. The bars are said to have a "front" face, or side, which faces upstream, i.e., faces the incoming flow of the liquid, the settleable solids, and the debris; and a "back" face, or side, which faces downstream, i.e., faces the exiting flow of the liquid and the settleable solids.
One type of removal facility is provided with very short rake tines which extend from the front face only partially into the openings between the bars, and not past the back face. These rake tines scrape the front face to remove the debris caught on the front face. The bars have lateral supports which extend across the back face of the bars to keep the bars properly spaced without interfering with the tines. This is a "front rake" removal facility, which performs a front raking operation.
The other type of removal facility is provided with much longer rake tines which extend from the back face completely through the openings between the bars and past the front face by a distance (such as a four to five inches) to be able to support and remove the debris caught on the front face. These bars do not have such lateral supports because such supports would interfere with the longer tines extending through the openings. This is the "back rake" type of removal facility, which performs a back raking operation.
In Applicants' experience, there are no removal facilities available which are readily convertible, or easily usable, or simultaneously usable, for both front raking and back raking. Therefore, it appears that a basin design engineer must either specify a front rake-type, or a back rake-type, of removal facility for a particular channel. Each such front rake-type or back rake-type of removal facility is different. This results in problems, first for the rake manufacturer because, for example, each type of rake must be separately inventoried if fabricated before an order is received, or must be separately fabricated to fill a specific order. In some situations, the engineer will not know which of the front rake-type, or back rake-type, removal facility will be best for the particular channel. Further, the engineer or liquid treatment plant operator may not know what type of debris will be present in the liquid in the channels, or if known at a given time, there may over time, e.g., from season to season, be changes in the type of debris present in the liquid in the channels.
As a result, if the wrong type of removal facility is installed, or if there is a change in the type of debris present in the incoming liquid in the channel, either less desirable screening will take place using the originally installed removal facility, or the originally installed removal facility must be modified for the new situation. Each of these alternatives is undesirable, increases the cost of operation of the plant, and requires time consuming removal of the installed rake from service.
The problems resulting from fixed bar screens having such great height above the channel, or which are significantly complex, or which tend to block the flow of the liquid and the suspended solids through the openings between the bars, or which are very costly, or which are not suitable for the type of debris currently to be removed from the liquid, are important drawbacks when the channels are large. However, since large size channels are generally part of large liquid treatment plants, e.g., plants which treat more than nine million gallons per day (9 mgd), more money may be available for equipment to solve these fixed bar screen problems than is generally available for such small size channels which supply liquid to smaller plants, e.g., plants which treat up to three mgd, which are built on a very low budget compared to the budgets of the large plants. Although these problems are common to removal facilities for both the large and the small liquid treatment plants, and the solutions of the present invention are applicable to removal facilities for channels for any size plant, there is greater incentive to have a cost-effective solution to these problems for the small plants.